This invention relates generally to the production of kinescopes for color television receivers and particularly to a method for measuring mask misregistry in the panel assemblies for such kinescopes.
The picture tube, or kinescope, for a color television receiver is manufactured by permanently coupling a funnel portion to a panel assembly. The funnel portion supports an electron gun which provides the electron beams needed to produce the color visual display. The panel assembly includes a phosphor screen and a shadow mask. The phosphor screen is composed of triads of phosphors each of which emits a different color of light when impacted by electrons. The shadow mask is spaced a predetermined distance, commonly called the Q spacing, from the screen. The shadow mask includes a large number of apertures through which the electron beams pass prior to reaching the phosphor screen. The apertures cause the electron beams from the individually modulated electron guns to impact phosphors of the proper light emitting color. The screen can also include a black matrix material which is used to separate the phosphors to improve the color purity of the visual output. The distance between the shadow mask and the electron guns therefore is important. This distance is commonly called the P distance. Basic geometry readily shows that a change in either the Q spacing, the P distance, or the lateral position of the mask relative to the phosphor screen, will cause a shift in the landing positions of the electron beams relative to the desired landing positions on the dots or lines of the phosphor screen. Such shifts in the landing positions cause a defect in the final product which is commonly referred to as misregister.
The phosphor screen is produced by coating the entire internal surface of the faceplate panel with a slurry containing a phosphor and a photosensitive material which sets upon exposure to light. After the slurry is applied, the shadow mask is inserted into the panel and the panel assembly is placed on an illumination mechanism, commonly called a lighthouse. The lighthouse includes an illumination source, typically a mercury vapor lamp, which is spaced the P distance from the shadow mask. Light from the illumination source passes through a lensing system of the lighthouse, and, through the apertures in the shadow mask to expose the slurry on the faceplate panel. The lensing system is used to cause the light to follow essentially the same paths that the electron beams follow during the operation of the kinescope. Accordingly, when the phosphor being exposed is the green light emitting phosphor, the lensing system is arranged to simulate the path of the green electron beam of the finished kinescope. Similarly, when the red or blue light emitting phosphor is being exposed, the lensing system is adjusted so that the light path simulates the path of the red or blue electron beam. The lighthouse optical system lighthouse includes a trimmer mechanism which includes a trimmer lens for each of the three colors of light emitting phosphors. The lighthouse optical system thus causes the light to simulate the path of the red, green or blue electron beams, depending on the adjustment of the trimmer mechanism.
During exposure, the phosphor slurry which receives light through the apertures is set, while that which is shaded by the metal portions between the apertures is not. For many types of panels, during exposure the panel assembly is moved so that continuous phosphor lines are formed. The shadow mask is removed from the panel and the unexposed slurry is washed away leaving the exposed phosphor in the desired areas. A photosensitive slurry containing another of the three phosphors is applied to the entire inside surface of the panel and the shadow mask reinserted. The trimmer mechanism is adjusted for the second color slurry and the exposure process repeated. Thus, the shadow mask is repeatedly inserted into, and removed from the panel. Also, for black matrix types of kinescopes, additional insertion and removal of the shadow mask is required.
The relative positions of the phosphors and the shadow mask apertures is commonly called registry. Thus, for a panel assembly having proper registry, the relative positions of the three color phosphors and the shadow mask apertures are aligned so that the individual electron beams impact the phosphors which emit the desired color of light. When the phosphors and the shadow mask apertures are misregistered, the centers of the electron beams will not coincide with the centers of their respective phosphor dots or stripes. Portions of the beam will therefor land on the guard bands surrounding the phosphor dots or stripes. The guard bands do not contain phosphor, and usually are filled with a black matrix material. Accordingly, the portion of the beam landing on the guard band will not produce fluorescence. Regions of the panel in which this occurs are darker than properly registered regions. If the misregister is great enough, portions of the electron beam can spill past the guard bands onto an adjacent phosphor. Regions of the panel in which this occurs have incorrect coloration. Both of these effects are objectional and typically result in the rejection of the kinescope.
Misregister can be measured in terms of the size and sign of the misregister, that is, an effective lateral displacement of the shadow mask relative to the panel. Misregister can also be measured in regard to the area of the panel over which the misregister occurs. The severity of a misregister defect is dependent on both of these factors. A small area, small magnitude defect may not be noticable by the end user, while either a small area, large magnitude or a large area, small magnitude defect might be easily seen. Small area defects are typically called local defects, and are usually caused by localized damage to the shadow mask. Large area defects are typically called global defects. These defects are generally caused by a shift, or rotation, of the shadow mask relative to the phosphor screen, or by a warp or bend of the mask as a whole.
The production of the phosphor screen on a faceplate panel is among the early processing steps in the production of a completed kinescope. Accordingly, a large number of expensive steps are carried out subsequent to the production of the phosphor screen. The detection of misregistry in a completed tube results in the scrapping of a very expensive tube. For these reasons, the detection of misregistry prior to joining the panel assembly and the funnel assembly together is very important.
A very effective method for detecting local misregistry is described in copending application Ser. No. 671,128 entitled "Method of Testing A Panel Assembly of a Cathode Ray Tube" filed Nov. 13, 1984 by James R. Matey. With this method, the trimmer lens for one of the three colors is replaced by an ultraviolet filter which is transparent to ultraviolet, but opaque to visible light. A completed panel assembly is exposed through the ultraviolet filter. When the panel registry is correct the ultraviolet light passes through the shadow mask and impacts the phosphor of the desired color, and the phosphor fluoresces with that color. When a large local misregistry condition, such as a dent, exists, the UV impacts the wrong phosphor and a change of color in misregistered areas results. This change of color is easily detectable by visual inspection.
The method described in the referenced copending application is very effective in detecting local misregister large enough to cause incorrect coloration. It is less effective in detecting global misregister, or small amounts of local misregister. As noted above, small amounts of misregister can cause an intensity variation, rather than a color variation. These intensity variations are much more difficult to detect by eye than the color variations for several reasons. The color shift is from one definite color to another, for example, from green to red or blue. The observer needs only to make a qualitative judgement. Evaluation of intensity variations requires a quantitative judgement. Additionally, most panels have an aluminum coating applied over the phosphors. Light from the phosphors which would go out the back of the tube is reflected toward the front of the tube. When inspecting such aluminized panels using the techniques described herein and in the referenced copending application, reliance is placed on some fraction of the UV light being able to penetrate the aluminum and reach the underlying phosphor. The aluminum coating is typically non-uniform and therefore different amounts of UV penetrate the aluminum, thereby causing intensity variations in the resulting fluorescence. Such variations are not related to misregister. The electron beams are much more penetrating than the UV light and the non-uniformity in the aluminization therefore has little, or no, detrimental effect on a finished tube.
For these reasons there is a need for a method for measuring misregistry in a kinescope panel assembly independent of human observation. The present invention fulfills this need.